Method and system for low-temperature air separation

ABSTRACT

A method for low-temperature air separation, in which an air-separation system having a column system is used that has a first column, a second column, a third column, and a fourth column, wherein fluid from the first column is fed at least into the second column, fluid from the second column is fed at least into the third column, fluid from the third column is fed at least into the fourth column, and fluid from the fourth column is fed at least into the third column, and wherein the fluid fed from the third column into the fourth column includes at least a portion of a side flow, which is withdrawn from the third column and has a lower oxygen content and a higher argon content than the third sump liquid. The present invention also relates to a corresponding system.

The present invention relates to a method and a system for low-temperature air separation according to the preambles of the independent claims.

PRIOR ART

The production of air products in the liquid or gaseous state by low-temperature separation of air in air-separation units is known and is described, for example, in H.-W. Haring (editor), Industrial Gases Processing, Wiley-VCH, 2006—in particular, section 2.2.5, “Cryogenic Rectification.”

Air-separation systems have rectification column systems, which, conventionally, can be designed, for example, as two-column systems—in particular, as traditional Linde double-column systems—but also as three-column or multi-column systems. In addition to the rectification columns for extracting nitrogen and/or oxygen in the liquid and/or gaseous state, i.e., the rectification columns for nitrogen-oxygen separation, rectification columns for extracting further air components—in particular, the noble gases krypton, xenon, and/or argon—can be provided. Frequently, the terms, “rectification” and “distillation” as well as “column” and “tower” or terms composed therefrom, are used synonymously.

The rectification columns of the mentioned rectification column systems are operated at different pressure levels. Known double-column systems have what is known as a high-pressure column (also referred to as a pressure column, medium-pressure column, or lower column) and what is known as a low-pressure column (also referred to as an upper column). The high-pressure column is typically operated at a pressure level of 4 to 7 bar—in particular, ca. 5.3 bar. The low-pressure column is operated at a pressure level of typically 1 to 2 bar—in particular, ca. 1.4 bar. In certain cases, higher pressure levels can also be used in both rectification columns. The pressures indicated here and below are absolute pressures at the head of the respective columns indicated.

In addition to gaseous, high-purity nitrogen and possibly oxygen which is as particle-free as possible, the supply with comparatively small amounts of gaseous argon is increasingly also desired—in particular, for supplying semiconductor fabrication plants (so-called fabs). For this purpose, either liquid argon can be delivered and evaporated on-site, or gaseous argon can be extracted on-site. The delivery of liquid argon entails not only economic disadvantages (transport costs, transfer losses, cold losses during evaporation against ambient air), but also places high demands on the reliability of the logistics chain. For the fields of application mentioned, systems for the low-temperature separation of air, which can deliver smaller amounts of gaseous argon in addition to larger amounts of gaseous, high-purity nitrogen, are therefore increasingly in demand. The nitrogen produced should typically have only ca. 1 ppb, and at most 1,000 ppb, oxygen, be substantially particle-free, and be deliverable at a pressure level significantly above atmospheric pressure. Specifications in ppb or ppm here refer to the mole fraction.

For argon extraction, air-separation systems with double-column systems and so-called crude- and, optionally, so-called pure-argon columns are typically used. An example is illustrated in Haring (see above) in FIG. 2.3A and described starting on page 26 in the section, “Rectification in the Low-pressure, Crude and Pure Argon Column,” and also starting on page 29 in the section, “Cryogenic Production of Pure Argon.” In principle, a pure-argon column can also be dispensed with in corresponding systems if the rectification columns in question are designed accordingly. Pure argon can then typically be withdrawn from the crude-argon column or a comparable column slightly further below the fluid conventionally transferred into the pure-argon column.

Even if only comparatively small amounts of argon are demanded, a complete air-separation system (i.e., equipped with a traditional low-pressure column for oxygen extraction) with double column and argon rectification must still conventionally be installed for the production of the gaseous argon, as explained above. The production of nitrogen at a pressure level significantly above atmospheric pressure with, simultaneously, large production quantities is not possible with reasonable yields in such systems. Here, the nitrogen is largely produced as a low-pressure product and must be compressed. The remaining part can be obtained under pressure-column pressure, but must also be re-compressed in most cases. In alternative system configurations in which only the high-pressure column is used for nitrogen production, the compression of nitrogen can indeed be omitted from the low-pressure column, but not the re-compressor. Moreover, nitrogen yields are generally poor in this case, and corresponding systems are also not well suited to argon production.

The aim of the present invention is therefore to specify a method and an air-separation system by means of which, in addition to relatively large quantities of high-purity, gaseous nitrogen at a pressure level significantly above atmospheric pressure, argon can also, advantageously, be provided.

DISCLOSURE OF THE INVENTION

Against this background, the present invention proposes a method and a system for low-temperature air separation, having the features of the independent claims. Preferred embodiments form the subject matter of the dependent claims and the following description.

Prior to explaining the features and advantages of the present invention, some of the principles of the present invention are explained in greater detail, and terms used below are defined.

The devices used in an air—separating system are described in the cited technical literature—for example in Häring (see above) in section 2.2.5.6, “Apparatus.” Unless the following definitions differ, reference is therefore explicitly made to the cited technical literature for the purpose of terminology used within the framework of the present application.

Liquids and gases can, in the terminology used herein, be rich or poor in one or more components, wherein “rich” can refer to a content of at least 75%, 90%, 95%, 99%, 99.5%, 99.9%, or 99.99%, and “poor” can refer to a content of at most 25%, 10%, 5%, 1%, 0.1%, or 0.01% on a molar, weight, or volume basis. The term, “predominantly,” may correspond to the definition of “rich.” Liquids and gases may also be enriched in or depleted of one or more components, wherein these terms refer to a content in a starting liquid or a starting gas from which the liquid or gas has been extracted. The liquid or the gas is “enriched” if it contains at least 1.1 times, 1.5 times, 2 times, 5 times, 10 times, 100 times, or 1,000 times the content, and “depleted” if it contains at most 0.9 times, 0.5 times, 0.1 times, 0.01 times, or 0.001 times the content of a corresponding component, relative to the starting liquid or the starting gas.

If, by way of example, reference is made here to “oxygen,” “nitrogen,” or “argon,” this is also understood to mean a liquid or a gas which is rich in oxygen, nitrogen, or argon, but need not necessarily consist exclusively of it.

The present application uses the terms, “pressure range” and “temperature range,” to characterize pressures and temperatures, which means that corresponding pressures and temperatures in a corresponding system do not have to be used in the form of exact pressure or temperature values in order to realize the inventive concept. However, such pressures and temperatures typically fall within certain ranges, which are, for example, ±1%, 5%, 10%, or 20% around an average. In this case, corresponding pressure ranges and temperature ranges can be in disjoint ranges or in ranges which overlap one another. In particular, pressure ranges include, for example, unavoidable or expected pressure losses. The same applies to temperature ranges. The values indicated here in bar relating to the pressure ranges are absolute pressures.

Where “expansion machines” are mentioned here, these refer to typically known turboexpanders. These expansion machines can, in particular, also be coupled to compressors. These compressors can, in particular, be turbocompressors. A corresponding combination of turboexpander and turbocompressor is typically also referred to as a “turbine booster.” In a turbine booster, the turboexpander and the turbocompressor are mechanically coupled, and the coupling can take place at the same rotational speed (for example, via a common shaft) or at different rotational speeds (for example, via a suitably geared transmission). In general, the term, “compressor,” is used herein. Here, a “cold compressor” refers to a compressor to which a fluid flow is supplied in a temperature range significantly below 0° C.—in particular, below −50, −75, or −100° C., and as far as −150 or −200° C. A corresponding fluid flow is cooled, in particular, by means of a main heat exchanger (see below) to a temperature in this temperature range.

A “main air compressor” is characterized in that it compresses all of the air supplied to the air-separation system and separated there. In contrast, only a portion of this air already previously compressed in the main air compressor is further compressed in one or more optionally-provided, further compressors—for example, post-compressors. Accordingly, the “main heat exchanger” of an air-separation system represents the heat exchanger in which at least the predominant portion of the air supplied to the air-separation system and separated there is cooled. This takes place at least in part in counterflow to material flows which are discharged from the air-separation system. In the language used herein, such “discharged” material flows or “products” are fluids which no longer participate in circuits within the system, but are permanently removed therefrom.

A “heat exchanger” for use in the context of the present invention can be designed in a customary manner. It is used to transfer heat indirectly between at least two fluid flows conducted, for example, in counterflow to each other, e.g., a warm, compressed air flow and one or more cold fluid flows, or a cryogenic liquid air product and one or more warm or warmer, but optionally also cryogenic, fluid flows. A heat exchanger can be formed from one or more heat exchanger sections connected in parallel and/or serially, e.g., from one or more plate heat exchanger blocks. This is, for example, a plate fin heat exchanger. Such a heat exchanger has “passages” which are designed as fluid channels, separated from one another, with heat exchange surfaces and are combined in parallel and separated by other passages to form “passage groups.” The characteristic of a heat exchanger is that heat is exchanged in it between two mobile media at one point in time, viz., at least one fluid flow to be cooled and at least one fluid flow to be heated.

A “condenser evaporator” refers to a heat exchanger in which a first, condensing fluid flow enters into indirect heat exchange with a second, evaporating fluid flow. Each condenser evaporator has a liquefaction chamber and an evaporation chamber. The liquefaction and evaporation chambers have liquefaction or evaporation passages. Condensation (liquefaction) of the first fluid flow is carried out in the liquefaction chamber, and evaporation of the second fluid flow in the evaporation chamber. The evaporation and liquefaction chambers are formed by groups of passages, which are in a heat-exchanging relationship with one another.

In a “forced-flow” condenser evaporator, a liquid flow or a two-phase flow is forced through the evaporation chamber by its own pressure and is partially or completely evaporated there. This pressure can be generated, for example, by a liquid column in the feed line to the evaporation chamber. The height of this liquid column here corresponds to the pressure loss in the evaporation chamber. In a “once-through” condenser evaporator of this type, the gas-liquid mixture emerging from the evaporation chamber, separated by phases, is forwarded directly to the next method step or to a downstream device, and, in particular, is not introduced into a liquid bath of the condenser evaporator, from which the portion remaining liquid would be sucked in again.

The relative spatial terms, “upper,” “lower,” “over,” “under,” “above,” “below,” “adjacent to,” “next to,” “vertical,” “horizontal,” etc., here refer to the spatial orientation of the rectification columns of an air-separation system or other components during normal operation. An arrangement of two components “one above the other” is understood here to mean that the upper end of the lower of the two components is located at a lower geodetic height than or the same geodetic height as the lower end of the upper of the two components, and the projections of the two apparatus parts overlap in a horizontal plane. In particular, the two components are arranged exactly one above the other, i.e., the axes of the two components run on the same vertical straight line. However, the axes of the two components need not lie exactly vertically one above the other, but can also be offset from one another—in particular if one of the two components, e.g., a rectification column or a column part of smaller diameter, is to have the same distance from the sheet-metal jacket of a cold box as another one of larger diameter.

ADVANTAGES OF THE INVENTION

Against this background, the present invention proposes a method for low-temperature air separation in which an air-separation system having a column system is used that has a first column, a second column, a third column, and a fourth column.

In the air-separation system according to the invention, the first to third columns emerge in particular from the addition of an additional column, operated at a higher pressure than the conventionally-present high-pressure column, to a traditional double-column system known from the prior art.

In the context of the present invention, as also explained in more detail with reference to the accompanying drawings, the first column can in particular be provided to be structurally separate from the second and third columns, wherein the second and the third columns can in particular be part of a double column and can be in heat-exchanging connection with one another by means of a corresponding condenser evaporator—the so-called main condenser. However, different arrangements of these can also be made; the present invention is not limited by the explanations just made.

In particular, an additional column can also be added to the second and third columns, which are designed as part of a double column, in a corresponding multiple-column system, or the second and third columns can be provided as separate columns. The aforementioned main condenser can be provided as an internal or external main condenser, as is known in principle from the prior art. When an internal main condenser is used, it is at least partially immersed in a sump liquid in the sump of the third column, and an overhead gas to be condensed from the second column is conducted through a condensation chamber of the main condenser.

As also known and customary in this respect in the field of air separation, a first sump liquid is formed in the first column, a second sump liquid is formed in the second column, a third sump liquid is formed in the third column, and a fourth sump liquid is formed in the fourth column. In contrast to the first through third columns, the fourth column is used in the context of the present invention in particular for argon production or argon discharge from a gas mixture removed from the third column. The fourth column can in particular be a conventional crude-argon column of a known arrangement with crude- and pure-argon columns, but it can also be a modified-argon column from which argon in a pure state is removed below the head, without the use of an additional pure-argon column. Other variants are also possible within the scope of the present invention.

In the context of the present invention, the first column is operated in a first pressure range, the second column is operated in a second pressure range below the first pressure range, and the third column is operated in a third pressure range below the second (and thus also the first) pressure range. The fourth column can in particular be operated in the third pressure range or in a pressure range slightly below it, which can in particular result from pressure losses over the lines connecting the third and fourth columns.

In the context of the present invention, the second sump liquid is formed with a higher oxygen content and a higher argon content than the first sump liquid, and the third sump liquid is formed with a higher oxygen content and a lower argon content than the second sump liquid. The higher argon content of the second sump liquid compared to that of the first sump liquid results from the different operating conditions—in particular, the different pressures used for operating the first and second columns—and from different compositions of material flows fed into the first and second columns. In contrast, the lower argon content in the third column results in particular from the fact that an argon-enriched gas is removed from this third column, as explained below.

In particular, in the context of the present invention, the first oxygen content can be 28 to 40%, and in particular ca. 34%, the second oxygen content can be ca. 45 to 65%, and in particular ca. 55%, and the third oxygen content can be ca. 99.0 to 99.9%, and in particular ca. 99.5%. The respective percentages relate to the molar content of oxygen in a corresponding component mixture. In the context of the present invention, the third column is therefore used as a pure-oxygen column, and a corresponding pure-oxygen product can be withdrawn from said column. In contrast, the first and the second sump liquids are typically not used as product, but are further prepared in the system.

In the context of the present invention, fluid is generally fed from the first column at least into the second column. The fluid fed from the first column into the second column can in particular comprise sump liquid of the first column, which is evaporated, expanded, and introduced into the second column. In particular, fluid can be fed from the first column into the second column and into the third column. Fluid from the second column is fed at least into the third column, fluid from the third column is fed at least into the fourth column, and fluid from the fourth column is fed at least into the third column.

If reference is made here in each case to “fluid” being fed from one column “at least” into another, this means, in particular, a direct or indirect transfer of a corresponding fluid flow. In particular, the transfer of corresponding fluid can also initially comprise the feeding into a condenser evaporator or its evaporation chamber, from which liquid and/or gaseous fractions are then transferred into the other column. Conducting fluid in this way thus also falls within the transfer of a fluid from one column into the other. The same also applies if a corresponding fluid is only partially transferred, e.g., if it is enriched with or depleted of specific components, and/or divided into partial flows.

The transferred fluids can comprise overhead gases, sump liquids, and/or side flows of corresponding columns. A “side flow” means a material flow which is withdrawn from a corresponding column between different separating trays or separating sections, whereas the overhead gas designates a gas mixture which is withdrawn from the column above the uppermost separating tray or separating region, and a sump liquid designates the liquid which is withdrawn from a corresponding column below the lowermost separating tray or separating region. A sump liquid is discharged in particular in the form of a liquid material flow, and an overhead gas is discharged in particular in the form of a gaseous material flow. However, liquid or gaseous material flows can also be withdrawn, for example, directly above the sump, but still below the lowermost separating section or the lowest separating tray. A side flow can be present in the liquid or gaseous state. A liquid side flow can be taken, for example, from a liquid retaining device or from an accumulation tray.

In the context of the present invention, the fluid fed from the third column into the fourth column comprises at least a portion of a side flow, which is withdrawn from the third column and has a lower oxygen content and a higher argon content than the third sump liquid. The side flow is withdrawn from the third column in particular in the region of the argon transition already explained above, but it can also be removed below the argon transition. A corresponding side flow is in particular a gas mixture which has a higher argon content than the sump liquid and a lower argon content than the overhead gas.

An essential aspect of the present invention is that a reflux liquid is formed by condensing overhead gas of the first column, and that this reflux liquid is fed back to the first column in liquid form. The formation of the reflux liquid “using” the overhead gas can comprise, in particular, withdrawing overhead gas in gaseous form from the first column, at least partially liquefying it in a condenser evaporator, which is also explained in detail below, and feeding back a liquid fraction at least partially to the first column.

According to the invention, for the explained condensation of the overhead gas of the first column, a liquid cooling flow is evaporated or partially evaporated in indirect heat exchange with the overhead gas. Gas formed during the evaporation or partial evaporation of the cooling flow (any remaining liquid can be separated out in advance) is further, according to the invention, expanded, so as to perform work, to a pressure in the second pressure range, i.e., the operating pressure of the second column, and fed into the second column. The correspondingly expanded gas can be a portion of the gas formed during evaporation or partial evaporation, or even only a portion thereof. The liquid cooling flow can in particular be provided at a pressure in the first pressure range and expanded to a slightly lower pressure, e.g., in a region between the first pressure range and the second pressure range, before evaporation or partial evaporation - for example, by means of a valve. A further expansion after evaporation to a pressure in the second pressure range before feeding into the second column then takes place downstream of the evaporation in the aforementioned work-performing form.

In an alternative, the present invention provides a method with lower energy consumption than previously developed methods. This applies in particular in comparison with known, so-called SPECTRA methods and variants thereof. However, the present invention can also be used in conjunction with such a SPECTRA method or a variant thereof, as explained in connection with FIG. 4, for example.

A SPECTRA method is known, for example, from EP 2 789 958 A1 and the patent literature cited therein. In the simplest form, this is a single-column method. However, a SPECTRA method can also be extended to oxygen and argon recovery, by providing further columns. SPECTRA methods enable a relatively high nitrogen yield.

From the rectification column for nitrogen extraction, which is also supplied with the main quantity of feed air, cryogenic liquid, enriched in oxygen compared with atmospheric air, is withdrawn in the form of one or more material flows and heated in the condenser evaporator, which is used for cooling and condensing at least a fraction of the overhead gas of the same column. Correspondingly condensed overhead gas is at least partially fed back to the column from which it was previously withdrawn. In conventional SPECTRA methods, the fluid to be evaporated can be conducted through the condenser evaporator in the form of only one material flow or in the form of two or more, separate, first material flows.

Part of the correspondingly evaporated fluid is cold-compressed, i.e., compressed at a temperature level significantly below 0° C.—in particular, at a temperature level of −50° C. or less—using one or more compressors and is then fed back into the column from which it was previously withdrawn. The one or more compressors can be coupled to one or more expansion machines—in particular, to one of two expansion machines arranged in parallel—and can at least partially be driven using the same. In the expansion machine or machines, a further portion of the evaporated fluid is expanded and discharged from the air-separation system.

In particular, the present invention is based upon the finding that the potential of a generic method is not fully exploited without the measures proposed according to the invention. An indicator of this is or was a still relatively high temperature difference present in such generic methods in a condenser which condenses overhead gas of the first rectification column operating at the highest pressure. In previous concepts, this condenser was designed, due to the requirements for the nitrogen product pressure and thus the first pressure level (for example, 11 bar; the feed air is correspondingly highly-compressed), as a bath condenser with a relatively high hydrostatic pressure loss of ca. 150-200 mbar and an average temperature difference of 2.5 K (or more). As a result, a relatively large amount of exergy is “lost” in a corresponding process, which is not the case within the scope of the present invention.

A further issue (seen as a disadvantage) of alternative concepts not according to the invention, which are based upon a process topology differing from SPECTRA methods and having multiple rectification columns, is the injection, intended to generate refrigeration capacity, of air (ca. 10-15% of the quantity of feed air) into the second rectification column operating at medium pressure.

This quantity of air is not subjected to the rectification in the first (main) rectification column in order to obtain high-pressure nitrogen product, and therefore a corresponding solution is inevitably associated with corresponding disadvantages in nitrogen yield.

As a result of the measures proposed according to the invention, a corresponding injection of feed air mainly provided for obtaining cold is no longer necessary, since the cooling fluid evaporated in the condenser evaporator can be used instead. This cooling fluid, advantageously, originates from sump liquid of the first rectification column and has therefore already participated in the rectification taking place there. This increases the efficiency and yield in the method proposed according to the invention.

In other words, the present invention proposes that a feed air flow into the second rectification column not (or at least not exclusively) be used to generate the process refrigeration capacity, but rather a flow evaporating in a condenser evaporator (overhead condenser) of the first rectification column, viz., the material flow referred to herein as the “cooling flow.” An essential aspect of the present invention consists, inter alia, in the exploitation of the differential pressure between the operating pressure of the second rectification column (i.e., the one corresponding to the second pressure range) and the evaporation pressure in the condenser used for condensing the overhead gas in order to generate the cold and to reduce the air “injection quantity” into the second rectification column.

The lower the temperature difference in the condenser is, the higher the evaporation pressure in said condenser (and thus the better for the cold generation process). The condenser is therefore advantageously designed not as a bath condenser, but in particular as a forced-flow condenser with the lowest possible minimal temperature difference.

In other words, in order to achieve the stated advantages, the aforementioned cooling flow (which is expanded instead of air for refrigeration capacity) is, advantageously, thus formed using at least a fraction of the first sump liquid, and/or a forced-flow condenser evaporator is, advantageously, thus used for condensing the overhead gas of the first column and for evaporating the cooling flow. With respect to the term, “forced-flow condenser evaporator,” reference is expressly made to the above explanations.

In a particularly preferred embodiment of the invention, which is thus part of a SPECTRA method, a further liquid cooling flow is, furthermore, evaporated or partially evaporated in indirect heat exchange with the overhead gas in order to condense the overhead gas of the first column, wherein the further cooling flow is withdrawn above the sump from the first column and is at least partially compressed after evaporation or partial evaporation and fed back to the first column. The compression of the evaporated, further cooling flow or a portion thereof takes place in particular in one or more compressors, which are mechanically coupled to one or more expansion machines and, in particular, are additionally braked. Compression takes place in particular at a temperature level below 0° C., and in particular below −50° C., e.g., at −100 to −150° C. The one or more expansion machines compress, in particular, a remainder of the cooling flow formed from sump liquid of the first column, which is not compressed and fed back to the second column. This compressed remainder is compressed, in particular, to a pressure in the first pressure range and fed back to the first column.

In the context of the present invention, the aforementioned first pressure range is in particular 9 to 12 bar, the second pressure range is in particular 4 to 6.5 bar, and the third pressure range is in particular 1 to 2 bar. However, the third pressure range can also be lowered further, and in particular by 50 to 200 mbar, for example, relative to the pressure value of 1.4 bar mentioned at the outset with respect to conventional air-separation systems. Accordingly, the second pressure range can also be lowered by 120 to 500 mbar—for example, relative to the aforementioned value of 5.3 bar. A corresponding decrease in pressure is possible, in particular, if, as explained below, gas from an evaporation chamber of a condenser evaporator, which condenses overhead gas of the fourth column, is used as regeneration gas. The driving temperature difference in this condenser evaporator is also reduced.

During the decrease in pressure to said values or the additional decrease in pressure, the refrigeration capacity can be correspondingly increased by increasing the pressure difference between the first pressure range and the second pressure range or by decreasing the outlet pressure during expansion. As mentioned, the pressure data here each denote absolute pressures at the head of the columns. The first pressure range is thus above a pressure which is conventionally used for a high-pressure column in an air-separation system. In the context of the present invention, the second column can in particular be operated at a lower pressure range than the high-pressure column conventionally used in an air-separation system. However, it can in principle also be the same pressure.

The use of the present invention - in particular, when the cooling flow used for condensing the overhead gas of the first column and evaporating the cooling flow is formed using the first sump liquid of the first column—means that, due to the pressure ratios present, no pump is necessary for conveying liquid nitrogen. This results in a lower heat input into the low temperature system, and, in contrast to the use of an oxygen-rich fluid, which is also possible in principle, there is a higher driving temperature difference owing to the lower oxygen content in the evaporating liquid.

This allows a higher evaporation pressure and correspondingly increases the turbine output.

The turbine output (process refrigeration capacity) achieved with the measures according to the invention is in particular sufficient for “gas” systems with a relatively small liquid production, e.g., in the case of a required nitrogen product pressure of 11 bar (which then also corresponds to the pressure in the first pressure range). In such a constellation, no further turbine is required. Furthermore, no injection of air into the second column or no bypass of the first column is required in this constellation. However, an additional turbine can nevertheless be used in an operating case with relatively high liquid production.

In the method proposed according to the invention, a nitrogen-rich gas is in particular withdrawn as a product from the first rectification column, heated, and discharged from the system at a pressure in the first pressure range. In this way, a nitrogen product can be provided in a corresponding pressure range without further compression.

Overall, in the context of the present invention, an energy advantage of 5-6%, relative to the output of the main air compressor, can be achieved, in comparison with the mentioned, likewise possible, expansion of feed air instead of the evaporated cooling flow, by increasing the yield of nitrogen product and the recovery of turbine output. This can correspond to a reduced energy consumption of, for example, ca. 500 kW and thus to a reduction in the TCO (Total Cost of Ownership) of more than 1 million EUR. Due to the measures proposed according to the invention, a smaller air feed quantity is required overall, which leads to a smaller “warm” system part. A smaller main heat exchanger (the kF value is ca. 6% lower with the same MTD value) can thus also be used. If a forced-flow condenser evaporator is used, combustible hydrocarbons are not enriched, due to the relatively high evaporation pressure (typically more than 7 bar).

The present invention can provide different possibilities for controlling or not controlling the temperature of the cooling flow, each of which can offer certain advantages in terms of energy. Thus, the gas, which is formed during the evaporation or partial evaporation of the cooling flow and expanded to perform work and fed into the second column, can be heated before expansion. In particular, the main heat exchanger of the air-separation system can be used for this purpose. In this way, feed flows can be cooled by means of corresponding refrigeration, and the heat exchanger profile can be adapted accordingly. However, the gas, which is formed during the evaporation or partial evaporation of the cooling flow and expanded to perform work and fed into the second column, can also be supplied to the expansion at a temperature at which it is present after evaporation or partial evaporation; thus, no further temperature control takes place in this embodiment.

In one embodiment of the present invention, which can likewise offer advantages in terms of energy under given boundary conditions, the gas, which is formed during evaporation or partial evaporation of the cooling flow and expanded to perform work and fed into the second column, can be fed into the second column at a temperature at which it is withdrawn from an expansion machine used for expansion. Alternatively, heating can also take place after expansion.

In the context of a particularly preferred embodiment of the present invention, overhead gas of the fourth column is condensed at least partially in a condensation chamber of a condenser evaporator, from the evaporation chamber of which a gas mixture is withdrawn.

In one embodiment of the present invention, part of this gas mixture and/or also at least part of the residual gas from the upper region of the third column, i.e., at least part of a gas mixture which is withdrawn from the third column, can be used to form a reflux flow and, in the process, heated, compressed, cooled, and fed into the second column. In the context of the present invention the main heat exchanger of the air-separation system, in particular, can be used for heating and cooling the aforementioned reflux flow.

In particular, a (further) portion of the gas mixture from the evaporation chamber of the condenser evaporator can also be used as regeneration gas for an adsorber in which feed air supplied to the column system is prepared. This can be done in particular at a corresponding evaporation pressure. The adsorber mentioned is operated in particular without the use of regeneration gas, which is withdrawn from the third column and fed to the adsorber in the same composition as there. However, gas from the upper region of the third column and gas from the lower region of this column can, for example, be combined and heated in the main heat exchanger as a common flow and used as regeneration gas.

In a particularly preferred embodiment of the invention, at least a portion of the gas mixture withdrawn from the evaporation chamber of the condenser evaporator is used as a first regeneration gas fraction, and at least a portion of the gas or of the mentioned gas mixture withdrawn from the third column is used as a second regeneration gas fraction. In this case, the second regeneration gas fraction, in particular, is provided at a lower pressure than the first regeneration gas fraction, compressed, and combined with the first regeneration gas fraction before it is fed to the adsorber. Residual gas from the third column and gas from an overhead condenser of the argon column are thus combined.

The embodiment just explained then has, in particular, advantages when no refrigeration machine is used in pre-cooling, and the regeneration gas requirement is therefore comparatively high. In such cases, the quantity of gas from the condenser evaporator can be insufficient as a regeneration gas quantity. A partial flow of the residual gas from the low-pressure column (i.e., the third column) is therefore re-compressed (the pressure difference is ca. 50 to 200 mbar) and merged with the other flow. Advantages result even if the requirements for the hydrogen content in the gaseous nitrogen product are comparatively high. In such cases, the air inlet temperature in the molecular sieve (i.e., the adsorber) cannot be selected to be arbitrarily low, since, otherwise, the hydrogen removal (on the special layer with catalyst provided for this purpose in the adsorber) is less complete. In such cases as well, the quantity of gas from the condenser evaporator may not suffice as the quantity of regeneration gas.

In a particularly preferred embodiment of the present invention, a fraction of the first sump liquid is supplied to the evaporation chamber of the condenser evaporator, in the condensation chamber of which the overhead gas of the fourth column is at least partially condensed, and subjected to partial evaporation, wherein, with this partial evaporation, the aforementioned gas mixture is formed. In other words, in this embodiment, an overhead condenser of a crude-argon column or of the single-argon column is therefore cooled using sump liquid of the first column. If a pure-argon column is present as a fifth column, its overhead condenser can also be cooled using corresponding sump liquid, as explained below.

In general, evaporated and unevaporated fractions of the sump liquid from the first column which were used in the condenser evaporator(s) of the fourth, or fourth and fifth, column(s) can then also be transferred at least partially into the third column (optionally, minus the fraction used in the adsorber), and in fact at a position corresponding to the oxygen content and argon content of these fluids. The evaporated and unevaporated fractions can therefore be fed into the third column at essentially the same point. Said material flows can be combined or transferred separately from one another into the third column. The fluid transferred from the first column into the third column thus comprises corresponding liquid, i.e., at least part of the first sump liquid which was used to cool the overhead condenser(s) of the fourth, or fourth and fifth, column(s). Optionally, feeding evaporated fractions from the overhead condenser(s) can also be omitted, and these evaporated fractions are discharged from the method without feeding into the third column, as mentioned in the case of the reflux flow and the fraction used in the adsorber. With the unevaporated liquid, however, a portion of the first sump liquid is fed into the third column in this case as well.

According to one embodiment of the present invention, the fluid fed from the second column into the third column can comprise at least a portion of the second sump liquid, which is transferred from the second column into the third column without the use of a pump. According to this embodiment, it is possible for corresponding sump liquid to be transferred to the third column solely owing to the pressure difference between the second and third columns. However, it can also be supercooled, beforehand or during transfer, against further flows, using a supercooling, counterflow heat exchanger.

Overhead gas can be withdrawn from the first column at a defined withdrawal position and discharged from the air-separation system as a compressed nitrogen product in a corresponding pressure range.

As already mentioned, the present invention can be used in combination with a pure-argon column, i.e., a fifth column, into which fluid from the fourth column is transferred, wherein the fluid transferred from the fourth column has an argon content which is higher than in the gas mixture withdrawn from the third column and transferred at least in part into the fourth column. In the context of the present invention, the fifth column is thus used to obtain a corresponding argon product, as is known in principle from the field of air-separation technology.

Advantageously, overhead gas of the fifth column is condensed by means of a further condenser evaporator, in which a further fraction of the second sump liquid is subjected to partial evaporation. Reference is explicitly made to the explanations above.

The side flow, formed, in the context of the present invention, with a lower oxygen content and a higher argon content than the third sump liquid and withdrawn from the third column, can be subjected to preparation in a further column—in particular, to obtain an oxygen-depleted gas mixture and an oxygen-rich liquid—wherein the oxygen-depleted gas mixture can be fed from the further column at least in part into the fourth column. In this way, part of the side flow reaches the fourth column via the deviation of the further column. The further column is in particular formed in two parts and comprises two parts arranged one above the other, which are separated by a fluid-tight separating tray, wherein the oxygen-depleted gas mixture is withdrawn at least at the head of the upper part, but, optionally, also at the head of the lower part, and the oxygen-rich liquid is withdrawn from the sump of the lower part. The side flow from the third column is fed—in particular, in gaseous form—into a lower region of the upper part. In particular, liquid is withdrawn from the sump of the upper part and fed back to the third column. Sump liquid of the fourth column is, in particular, delivered as a reflux at the head of the upper part, but can also be delivered in part as a reflux to the lower part. In such an arrangement, the lower part fulfills the function of a (high-purity) oxygen column. Different embodiments are illustrated in the figures.

Finally, the present invention also extends to an air-separation system, for the features of which reference is expressly made to the corresponding independent claim. In particular, such an air-separation system is designed for carrying out a method as explained above in different embodiments, and in each case has means designed for this purpose. For features and advantages of a corresponding air-separation system, reference is expressly made to the explanations relating to the method according to the invention.

The invention is explained in more detail below with reference to the accompanying drawings, which illustrate embodiments of the present invention and embodiments not according to the invention.

DESCRIPTION OF THE FIGURES

FIGS. 1 through 4 show air-separation systems, which correspond to embodiments of the present invention where they fall within the scope of protection of the patent claims, and otherwise relate to the technical background and/or embodiments not according to the invention. The air-separation systems according to FIGS. 1 through 4 are respectively designated as a whole by reference signs 100 through 400. Although the following explanations relate to corresponding air-separation systems 100 through 400, they relate to corresponding methods in the same way. The air-separation system 500 illustrated in FIG. 5 is shown as a variant of the air-separation system 100 illustrated in FIG. 1. The aspects illustrated here can nevertheless also be implemented by other systems—in particular, by systems 200 through 400. The following explanations—in particular, regarding the system 100 according to FIG. 1—relate to the system 500 in the same way, even if this reference is not specifically made.

All the air-separation systems 100 through 400 shown in FIGS. 1 through 4 are equipped with a column system which, irrespective of the different design and, optionally, different number of columns, is in each case designated overall by 10. The column systems 10 each have a first column 11, a second column 12, a third column 13, and a fourth column 14.

The second column 12 and the third column 13 are each designed as parts of a double column of a type known in principle. Reference is expressly made in this context to the technical literature cited at the outset regarding air-separation systems—in particular, to the explanations relating to FIG. 23A in Häring (see above), in which a corresponding double column is shown.

The first column 11 is formed separately from the second column 12 and from the third column 13. The first column 11 is equipped with a condenser evaporator 111, which is used for condensing overhead gas of the first column 11 and is designed as a traditional overhead condenser in the embodiments according to FIGS. 1 through 3. In each case, sump liquid, which is conveyed without the use of a pump, is fed from the first column 11 into the condenser evaporator 111, which is in each case designed as a forced-flow condenser evaporator in the examples shown.

An essential aspect of the embodiments of the invention illustrated here is in each case that a reflux liquid is formed by condensing overhead gas of the first column 11 and that the reflux liquid is fed back to the first column 11. In order to condense the overhead gas of the first column 11, a liquid cooling flow, which is formed using the mentioned sump liquid from the first column 11, is evaporated or partially evaporated with the overhead gas of the first column 11. Gas formed during the evaporation or partial evaporation of the cooling flow is expanded, so as to perform work, by means of an expansion machine 5 to a pressure in the second pressure range and fed into the second column 12.

The second column 12 and the third column 13 are connected to each other, so as to exchange heat, via an internal condenser evaporator 121—the so-called main condenser. The main condenser 121 is used, on the one hand, for condensing an overhead gas of the second column 12 and, on the other, for evaporating a sump liquid of the third column 13. As an alternative to the embodiment illustrated here, the second column 12 and the third column 13 can also be separate. The main condenser 121 can, alternatively, also be designed to be on the outside. Different types of condenser evaporators can be used as main condensers 121.

The fourth column 14 is used for argon production in all the air-separation systems 100 through 400 according to FIGS. 1 through 4. In the examples shown, no crude-argon column is present, but, rather, the systems 100 through 400 are each designed for withdrawal of an argon product from the fourth column 14. For crude- and pure-argon columns and corresponding modifications, reference is likewise made to the above citations from the technical literature.

The fourth column is equipped with a condenser evaporator (overhead condenser) 141 which condenses overhead gas. In the embodiments according to FIGS. 1 through 3, this is cooled with a portion of sump liquid from the first column 11, whereas sump liquid from the second column 12 is used for this purpose in the embodiment according to FIG. 4. The sump liquid used in each case is previously supercooled by a supercooling, counterflow heat exchanger 18. A fraction unevaporated in the overhead condenser 141 is at least partially fed into the third column 13 in the examples illustrated here. On the other hand, in the examples illustrated here, an evaporated fraction is used to regenerate an adsorber and, in the case of the air-separation system 300 according to FIG. 3, to form a reflux flow, as explained below.

In all the air-separation systems 100 through 400 according to FIGS. 1 through 4, a further column 15 is provided, in which a material exchange is carried out between a fraction of a sump flow from the fourth column 14 and a side flow from the third column 13, and a fraction of the sump flow from the fourth column 14 is depleted of highly-volatile components. The further column 15 has an upper and a lower region, which are, functionally, completely separated from one another. Further details are explained in each case below. The further column 15 is designed with a condenser evaporator 152 which is heated with overhead gas from the second column 12.

As a component directly associated with the column system 10, a pump 19 is present in all the air-separation systems 100 through 300 according to FIGS. 1 through 3 and conveys the sump liquid back from the fourth column 14 into the further column 15.

In all the air-separation systems 100 through 400 according to FIGS. 1 through 4, a sump liquid is formed in the first column 11 and is referred to here as first sump liquid. Accordingly, a second sump liquid is formed in the second column 12, a third sump liquid is formed in the third column 13, and a fourth sump liquid is formed in the fourth column 14. The first column 11 is operated in a first pressure range, the second column 12 is operated in a second pressure range below the first pressure range, and the third column 13 is operated in a third pressure range below the second pressure range. The second sump liquid is formed with a higher oxygen content and a higher argon content than the first sump liquid, and the third sump liquid is formed with a higher oxygen content and a lower argon content than the second sump liquid. Reference is made to the above explanations regarding the pressure ranges and oxygen or argon content.

In the manner explained below, in all the air-separation systems 100 through 400 according to FIGS. 1 through 4, fluid is fed from the first column 11 into the second column 12 (and also into the third column 13 in the air-separation systems 100 through 300 according to FIGS. 1 through 3). Furthermore, fluid is fed from the second column 12 into the third column 13, and fluid is fed from the fourth column 14 into the third column 13. In all the air-separation systems 100 through 400 according to FIGS. 1 through 4, the fluid fed into the fourth column 14 from the third column 13 comprises at least a portion of a side flow which is withdrawn from the third column 13 and has a lower oxygen content and a higher argon content than the second sump liquid. At least in the embodiments illustrated here, the other fluids mentioned each comprise at least portions of the respective sump liquids. In all cases, direct feeding or feeding via an intermediate overhead condenser or the like and corresponding partial feeding can take place.

In particular, the air-separation system 100 according to FIG. 1 is first explained in more detail below. For the sake of clarity, the explanations relating to the air-separation systems 200, 300, and 400 according to FIGS. 2 through 4 each relate only to the features deviating therefrom. In FIGS. 2, 3, and 4, identical features are also provided with corresponding reference symbols only in some cases.

In the air-separation system 100 according to FIG. 1, a feed air flow a from the atmosphere, which is generally designated A here, is introduced by means of a main air compressor 1 via a filter, which is indicated by crosshatching and without a separate designation, cooled in an aftercooler, which likewise has no separate designation, and supplied to a direct-contact cooler 2, which is operated with cooling water W.

After the pre-cooling takes place in the direct-contact cooler 2, the feed air flow still designated a is freed of water and carbon dioxide in an adsorption device 3 in a manner described multiple times in the literature. The adsorption device 3, also generally referred to as an “adsorber” above, can be regenerated by means of a regeneration gas flow z. The formation of the regeneration gas flow z is explained below.

The feed air flow, which is still denoted by a, correspondingly treated, and thus purified, is fed to the warm side of a main heat exchanger 4. The feed air flow a is withdrawn from the main heat exchanger 4 on the cold side or near its cold end and fed into the first column 11.

The sump liquid of the first column 11 is withdrawn therefrom and divided into two partial flows d and e in the air-separation systems 100 through 300 according to FIGS. 1 through 3. The partial flow d is here fed into the condenser evaporator 111 and evaporated. The evaporated partial flow d is then partially heated in the main heat exchanger 4 and then expanded to the operating pressure of the second column 12 in an expansion machine 5, which is coupled to a generator G, and fed into this second column 12 in a lower region. The treatment of the sump liquid in the air-separation system 400 according to FIG. 4 differs therefrom. Reference is made to the specific explanations below.

In contrast, in the air-separation systems 100 through 300 according to FIGS. 1 through 3, the partial flow e is conducted through the supercooling, counterflow heat exchanger 18 and then through the condenser evaporator 141. As illustrated in the form of a linkage f, a portion can also be fed into the second column 12. Gas formed in the overhead condenser 141 can be used as the aforementioned regeneration gas flow z. For this purpose, it is first heated in the supercooling, counterflow heat exchanger 18 and then in the main heat exchanger 4. As illustrated here in the form of a material flow g, a fraction which remains liquid is fed into the third column 13. In the air-separation system 400 according to FIG. 4, no corresponding partial flow e or f is formed. Reference is also expressly made here to the specific explanations below.

The overhead gas of the first column 11 is partially conducted through the condensation chamber of the overhead condenser 111 in the form of a material flow h and fed back to the first column 11 as a liquid reflux. A further fraction is heated in the form of a material flow i in the main heat exchanger 4 and, as a gaseous, compressed nitrogen product, discharged from the air-separation system 100 or used otherwise.

The sump liquid of the second column 12 is withdrawn therefrom in the form of a material flow j, conducted through the supercooling, counterflow heat exchanger 18, and fed into the third column 13. In the air-separation system 400 according to FIG. 4, the material flow j is conducted through the condenser evaporator 141 for cooling, as an alternative to the material flow e (see above). Gas formed in the overhead condenser 141 of the air-separation system 400 can also be used here as a regeneration gas flow, which is likewise designated z for the sake of simplicity. In the air-separation system 400 according to FIG. 4, gas is, furthermore, fed from the overhead condenser 141 or its evaporation chamber into the third column 13. As also illustrated in FIG. 4 in the form of a material flow g, a fraction which remains liquid is fed into the third column 13.

The overhead gas of the second column 12 is partially conducted in the form of a material flow k through the condensation chamber of the main condenser 121, liquefied there, and again fed back in part to the second column 12 as a liquid reflux. A further fraction is liquefied in the form of a material flow I in the condensation chamber of the condenser evaporator 152. In the air-separation systems 100 through 300 according to FIGS. 1 through 3, this further fraction is combined with the fraction liquefied in the condensation chamber of the main condenser 121, as illustrated in the form of the linkage I. Corresponding liquid can also be delivered by means of a pump 6 as reflux to the first column 11. In the embodiments according to FIGS. 1 through 3, the pump 6 conveys a liquid, nitrogen-rich flow b, which is withdrawn from the second column 12 in an upper region. In the examples according to FIGS. 1 through 3, a further fraction of overhead gas from the second column 12 is discharged from the system in the form of a material flow c.

In the embodiment of the air-separation system 400 according to FIG. 3, the liquefied fraction of the material flow k and of the material flow I are not combined. Rather, in this case, fractions of the material flow k are delivered separately from one another to the second column 12 and the third column 13 after liquefaction. The material flow I is fed separately into the third column 13.

The sump liquid of the third column 13 is withdrawn from it in the form of a material flow o, pressurized in liquid form by means of an internal compression pump 7, converted into the gaseous or critical state in the main heat exchanger 4 by heating, and discharged from the air-separation system 100 as a gaseous, compressed oxygen product or used otherwise. In contrast, gas withdrawn above the sump from the third column 13 in the form of a material flow p is combined with residual gas from the third column 13 (see below) to form a collective flow q, which is subsequently heated in the main heat exchanger 4 and discharged from the air-separation system 100 or used otherwise.

The overhead gas of the third column 13 is conducted through the supercooling, counterflow heat exchanger 18 in the form of a material flow r and, in the air-separation systems 100 through 300 according to FIGS. 1 through 3, combined with the material flow o to form the collective flow q, as mentioned. In the air-separation system 400 according to FIG. 4, a separate discharge takes place.

Furthermore, a side flow t is withdrawn in gaseous form from the third column 13 and first fed into an upper part of the further column 15. In contrast, a material flow u is fed back in liquid form from the upper part of the further column 15 into the third column 13. In the upper part of the further column 15, a mass transfer with sump liquid from the fourth column 14 is carried out, which, in the air-separation systems 100 through 300 according to FIGS. 1 through 3, is delivered in liquid form in the form of a material flow v into the upper and lower parts of the further column 15. In the air-separation system 400 according to FIG. 4, the material flow v is fed only into the upper part of the further column 15.

In the lower part of the further column 15, more volatile components are expelled through heating by means of the condenser evaporator 152. Gas is withdrawn from the upper and lower parts of the further column 15 and, in the air-separation systems 100 through 300 according to FIGS. 1 through 3, fed into the fourth column 14 in the form of a material flow w. In contrast, in the air-separation system 400 according to FIG. 4, the fourth column 14 is fed only with a gas flow w′ from the upper part of the further column 15. Gas and liquid exchange between the upper and lower parts of the further column 15 takes place here in the form of the material flows w″ and w′″. A portion of the side flow t is thus ultimately fed into the fourth column 14, and a portion of the sump liquid is ultimately fed back from it into the third column 13. In all the examples illustrated here, the further column 15 can, for example, also be arranged above the overhead condenser 111 of the first column 11.

Sump liquid from the lower part of the further column 15 is withdrawn in the form of a material flow x and, in the example shown, fed into a tank system T. If necessary, a material flow, also designated x for the sake of clarity, is withdrawn from the tank system T, evaporated in the main heat exchanger 4, and discharged as a high-purity, gaseous oxygen product.

Argon-rich liquid is withdrawn from the fourth column 14 in the form of a material flow y by means of a further internal compression pump 8, pressurized in liquid form, converted to the gaseous or critical state in the main heat exchanger 4 by heating, and discharged from the air-separation system 100 as a gaseous, compressed argon product or used otherwise. In the air-separation system 400 according to FIG. 4, a corresponding tank system T′ is also illustrated in this context.

Liquid nitrogen, liquid oxygen (optionally, also having different purities), and liquid argon can be provided as further products of the system 100, as is known in principle and shown, for example, in the form of a partial flow of the liquefied overhead gas h of the first column 11. In the embodiment of the air-separation system 400 according to FIG. 4, a liquid nitrogen feed into the condenser evaporator 111 is also shown using a material flow h′.

The air-separation system 200 illustrated in FIG. 2 and designed according to an embodiment of the invention differs from the air-separation system shown in FIG. 1 essentially in that the material flow d is not heated in the expansion machine 8 prior to its expansion.

The air-separation system 300 illustrated in FIG. 3 differs from the air-separation system 200 shown in FIG. 2 essentially in that a division of the material flow z is carried out on the warm side of the main heat exchanger 4, wherein a partial flow z′ of the material flow z is compressed by means of a compressor 9, cooled in the main heat exchanger 4, and fed into the second column 12. The air-separation system 300 can otherwise also be the same as the air-separation system 100 illustrated in FIG. 1.

The air-separation system 400 shown in FIG. 4 illustrates the measures proposed according to the invention in connection with a SPECTRA process known per se. In this case, in addition to the sump flow d, which has already been explained above, a further material flow d′ is withdrawn above the sump from the first column 11 and, like the material flow d, cooled again in the main heat exchanger 4. Evaporation then takes place in the condenser evaporator 111.

The material flow d is, in the expansion machine 5, which is coupled to a generator G, expanded in part and, as mentioned, fed into the second column 12 (see linkage D). The remainder of the material flow d is partially heated in the main heat exchanger 4 and then expanded in a further expansion machine 401, which is coupled to a compressor 402 and a brake 403. Discharge from the air-separation system 400 then takes place. A portion of the liquefied material flow h is discharged in liquid form and, optionally, supercooled against a portion of the same material flow in a supercooler 404. The fraction used for supercooling can be combined with the expanded remainder of the material flow d.

In contrast, after its evaporation in the condenser evaporator 111, the material flow d′ is at least partially subjected to compression in the compressor 402, cooled again in the main heat exchanger 4, and fed back to the first column 11.

As mentioned, the air-separation system 500 shown in FIG. 5 is illustrated as a variant of the air-separation system 100 according to FIG. 4. It is characterized in particular by the use of a blower 501, by means of which a portion of the material flow q, here denoted by q′, is brought to the pressure of the material flow z and fed to the latter.

As mentioned, such an embodiment is particularly advantageous when no refrigeration machine is used in the precooling of the feed air, and the regeneration gas requirement is therefore comparatively high. Advantages result even if the requirements for the hydrogen content in the nitrogen product are comparatively high. Reference is made to the corresponding explanations above. 

1-15 (canceled)
 16. A method for low-temperature air separation, in which an air-separation system having a column system is used that has a first column, a second column, a third column, and a fourth column, wherein a) a first sump liquid is formed in the first column, a second sump liquid is formed in the second column, a third sump liquid is formed in the third column, and a fourth sump liquid is formed in the fourth column, b) the first column is operated in a first pressure range, the second column is operated in a second pressure range below the first pressure range, and the third column is operated in a third pressure range below the second pressure range, c) the second sump liquid is formed with a higher oxygen content and a higher argon content than the first sump liquid, and the third sump liquid is formed with a higher oxygen content and a lower argon content than the second sump liquid, d) fluid from the first column is fed at least into the second column, fluid from the second column is fed at least into the third column, fluid from the third column is fed into the fourth column, and fluid from the fourth column is fed at least into the third column, and e) the fluid fed from the third column into the fourth column comprises at least a portion of a side flow which is withdrawn from the third column and has a lower oxygen content and a higher argon content than the third sump liquid, wherein f) a reflux liquid is formed by condensing overhead gas of the first column, and the reflux liquid is fed back to the first column in liquid form, g) in order to condense the overhead gas of the first column, a liquid cooling flow is evaporated or partially evaporated in indirect heat exchange with the overhead gas, and h) gas formed during the evaporation or partial evaporation of the cooling flow is expanded, so as to perform work, to a pressure in the second pressure range and fed into the second column.
 17. The method according to claim 16, in which the cooling flow is formed using at least a fraction of the first sump liquid.
 18. The method according to claim 17, in which a further liquid cooling flow is evaporated or partially evaporated in indirect heat exchange with the overhead gas in order to condense the overhead gas of the first column, wherein the further cooling flow is withdrawn above the sump from the first column and at least partially compressed after evaporation or partial evaporation and fed back to the first column.
 19. The method according to claim 16, in which a forced-flow condenser evaporator is used for condensing the overhead gas of the first column and for evaporating the cooling flow.
 20. The method according to claim 16, in which the gas, which is formed during the evaporation or partial evaporation of the cooling flow and expanded and fed into the second column, is heated before the expansion.
 21. The method according to claim 16, in which the gas, which is formed during the evaporation or partial evaporation of the cooling flow and expanded to perform work and fed into the second column, is supplied to the expansion at a temperature at which it is present after evaporation or partial evaporation.
 22. The method according to claim 16, in which the gas, which is formed during the evaporation or partial evaporation of the cooling flow and expanded to perform work and fed into the second column, is fed into the second column at a temperature at which it is withdrawn from an expansion machine used for expansion.
 23. The method according to claim 16, in which overhead gas of the fourth column is at least partially condensed in a condensation chamber of a condenser evaporator, from the evaporation chamber of which a gas mixture is withdrawn, wherein at least a portion of the gas mixture withdrawn from the evaporation chamber of the condenser evaporator is used to form a reflux flow, which is heated, compressed, cooled, and fed into the second column and/or is used as regeneration gas for an adsorber in which feed air is prepared which is fed to the column system is prepared.
 24. The method according to claim 16, in which at least a portion of a gas mixture which is withdrawn from an upper region of the third column is used to form a reflux flow which is heated, compressed, cooled, and fed into the second column.
 25. The method according to claim 23, in which the adsorber, in which the feed air which is fed to the column system is prepared, is operated without the use of regeneration gas, which is withdrawn from the third column and fed to the adsorber in a materially-unchanged composition.
 26. The method according to claim 23, in which at least a portion of the gas mixture withdrawn from the evaporation chamber of the condenser evaporator is used as a first regeneration gas fraction, and in which at least a portion of the gas withdrawn from the third column is used as a second regeneration gas fraction, wherein the second regeneration gas fraction is provided at a lower pressure than the first one, compressed, and combined with the first regeneration gas fraction before it is fed to the adsorber.
 27. The method according to claim 16, in which power released during work-performing expansion of the evaporated or partially evaporated cooling flow, or at least its fraction which is expanded to perform work and fed into the second column, is used for operating an electric generator.
 28. The method according to claim 16, in which the second pressure range is 4 to 6.5 bar.
 29. The method according to claim 16, in which the side flow formed with a lower oxygen content and a higher argon content than the third sump liquid and withdrawn from the third column is subjected to preparation in a further column to obtain an oxygen-depleted gas mixture and a sump liquid, wherein the oxygen-depleted gas mixture is fed from the further column at least in part into the fourth column.
 30. An air-separation system which has a column system having a first column, a second column, a third column, and a fourth column and is configured a) to form a first sump liquid in the first column, to form a second sump liquid in the second column, to form a third sump liquid in the third column, and to form a fourth sump liquid in the fourth column, b) to operate the first column in a first pressure range, to operate the second column in a second pressure range below the first pressure range, and to operate the third column in a third pressure range below the second pressure range, c) to form the second sump liquid with a higher oxygen content and a higher argon content than the first sump liquid, and to form the third sump liquid with a higher oxygen content and a lower argon content than the second sump liquid, d) to feed fluid from the first column at least into the second column, to feed fluid from the second column at least into the third column, to feed fluid from the third column at least into the fourth column, and to feed fluid from the fourth column at least into the third column, and e) to use at least a portion of a side flow which is withdrawn from the third column and has a lower oxygen content and a higher argon content than the third sump liquid as the fluid fed from the third column into the fourth column, wherein means are configured f) to form a reflux liquid by condensing overhead gas of the first column, and to feed back the reflux liquid to the first column in liquid form, g) to evaporate or partially evaporate a liquid cooling flow in indirect heat exchange with the overhead gas in order to condense the overhead gas of the first column, and, h) to expand gas formed during the evaporation or partial evaporation of the cooling flow, so as to perform work, to a pressure in the second pressure range and to feed it into the second column. 